The usefulness of the phenomenon of magnetostrictivity in linear distance or position measuring devices is well documented in the prior art. The basic technology comprises a taut magnetostrictive wire supported usually in a tube, a magnet movable along the tube, a circuit for applying an electrical pulse to the wire, and a sensor at one end of the wire for sensing a sonic pulse emitted at the magnet due to an interaction between the magnetic field and the electrical pulse. A timing circuit measures the time between the electrical pulse and the sensed sonic pulse to determine the propagation time of the pulse which is proportional to the distance between the magnet and the sensor. A common problem in the implementation of such a device is that spurious signals due to electrical noise or mechanical shock imparted to the probe masquerade as sonic pulses and thus corrupt measurements of the true propagation time. The U.S. Pat. No. No. 4,721,902 to Tellerman et al entitled "Noise Reduction in a Sonic Transducer" discloses the basic type of position detector and further discusses the problem of noise. That patent proposes to reject such noise by generating an inhibit signal during much of the time between the excitation pulse and the receipt of the true sonic pulse so that any intervening noise will be ignored; generation of that inhibit signal requires an a priori knowledge of the approximate position of the movable magnet and a substantial margin to allow the movement of the magnet between measurements. It is desirable to have a noise rejection method which operates independently of the position of the movable magnet.
One improvement over the basic technology of particular interest is the increase in measurement resolution as set forth in the U.S. Pat. No. 5,017,867 to Dumais et al entitled "Magnetostrictive Linear Position Detector with Reflection Termination", which is incorporated herein by reference. According to that patent a magnetostrictive wire is stretched between a head and a reflective termination and a displaceable magnet is movably disposed along the wire in accordance with the position to be detected. The wire is excited by an electrical pulse which interacts with the field of the magnet to induce a torsional motion in the wire which propagates as a sonic pulse in one direction directly to the head and a pulse in the opposite direction to the termination where it is reflected to the head. A detector at the head senses the arrival times of the pulses which are spaced in time according to the position of the magnet and the propagation velocity along the wire. The difference of the arrival times and the known length of the wire are used to calculate the position of the magnet. Alternatively, the wire is excited by a sonic pulse at the head and an electrical pulse is induced by the magnet when the mechanical pulse reaches the magnet directly and again upon reflection. The timing of the electrical pulses is used to calculate the magnet position in the same way as the electrical excitation type of detector. The Dumais et al improvements are also vulnerable to noise in the system but they offer the basis of a solution: a fixed reference length along the wire which is useful as a test for signal validity. The known length of the wire itself is available to use as such a reference length.
Another related apparatus offering a known reference length is disclosed by McCrea et al U.S. Pat. No. 4,158,964 and Redding 4,305,283. Both of these patents disclose a magnetostrictive detector with top and bottom reference magnets in a tank to afford an approximately fixed reference, and an intermediate movable magnet. Sonic pulses from the magnet yield ratiometric time periods which are used to determine the position of the movable magnet. There is no teaching of noise rejection using such a probe.